A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core plate and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. The top guide includes several openings formed by cross members, sometimes collectively referred to as a grid, and fuel bundles are inserted through the openings and are supported by the core plate. The top guide is supported vertically and horizontally by the shroud, and the top guide grid restrains horizontal movement of the upper ends of the fuel assemblies. Under normal operating conditions, there are no substantial vertical loads applied to the grid.
Irradiation is know to assist in the cracking of some BWR materials. The grid is located in a high fluence region and may be susceptible to irradiated assisted stress corrosion cracking (IASCC). The core support structure is exposed to the highest fluence. The core plate also is in a high fluence region and may be susceptible to IASCC.
The top guide grid must be removed if it becomes necessary to replace the top guide or the core plate. In some known BWRs, however, the grid is integral with the upper shroud assembly. Therefore, the grid cannot be replaced without removing the entire upper shroud assembly. Removing the entire upper shroud assembly requires removing approximately 90 stud/nut/keeper assemblies and disconnecting two to four core spray piping lines.
Other known BWRs have removable top guide grids. Specifically, the grid is secured to other components of the shroud assembly by bolted joints. Vertical loads are carried by the preload of the bolts, and horizontal loads are carried by shear pins and/or friction in the joint. Assembling such top guides is a tedious and time consuming task. Specifically, the grid must be accurately aligned so that the shear pins can be properly positioned, and each bolt must be torqued to the required pre-load.
It would be to desirable to provide a top guide grid which can be easily removed and replaced. It also would be desirable to provide such a grid which is easy to install.